A method for coating shapes having a tridimensional surface to be coated, by the cathode sputtering of target material is known (German Patent 31 07 914), to which U.S. Pat. No. 4,426,267 corresponds wherein the shape or shapes are exposed simultaneously, on the side opposite the cathode system, to the sputtering of a second cathode system with the same target material and with concentration of a second discharge space adjacent the target surface by a second magnetic field that is spatially closed with respect to the target. In this method a substrate bias is used, but it has to be within fixed limits considering the degree of ionization of the plasma, so that the substrate bias current per unit area has to be limited to less than 1 mA/cm.sup.2. The substrate bias current can be increased only by increasing the substrate bias voltage. This, however, has a negative influence on the tension in the coatings, so that the applied coatings can easily break off.
For vacuum cathode sputtering processes magnetron cathodes are used so that a sufficient coating rate can be achieved. It is not possible by this method to coat three-dimensional substrates without providing for a complex rotatory movement of the substrates. The solution of this problem is disclosed in U.S. Pat. No. 4,426,267 in which a double cathode arrangement of these magnetic sources has been published. The use of this arrangement is limited, however, to a narrow range of dimensions of the substrate, especially because the degree of ionization of the plasma of the two magnetron cathodes cannot be affected by process parameters that have to be adjusted separately.
The degree of ionization is of crucial importance to the quality of the coating, because in most applications the achievement of ionic plating conditions is indispensable. For the double cathode arrangement this signifies that the magnetrons have to be arranged so that their plasma zones overlap and surround the substrate. Only then is it assured that the substrates, when they are given a negative bias, will be bombarded by inert gas ions in a manner similar to the target, in order to have a positive influence on the structures that form as the coating grows. Even under these conditions, however, the ion bombardment is not isotropic, so that differences in quality become visible on the parts being coated. The coating quality could be improved by increasing the bias current per unit area. This, however, can be achieved only within narrow limits in the conventional arrangement. A way of increasing the bias current might consist in increasing the bias applied during the coating. This change will, however, simultaneously increase the energy of the impinging ions, so that consequences must be expected as regards the coating structure and properties. It is known that particularly the internal tensions present in the coatings can be influenced by the bias voltage. Another way of increasing the ionization is to go to higher cathode outputs. This possibility exists because the degree of ionization depends on the intensity of the secondary electron emission in the cathode sputtering process. However, it is not possible for technical reasons to increase the cathode output without limit, especially because the design of the cathode cooling permits only a narrow range. For the reasons described, therefore, there is need for a way to achieve an increase of the bias current per unit area at the lowest possible bias voltage. To improve the all-around coating of substrates and increase the substrate current in cathode sputtering apparatus, especially for hard coatings, different magnetron cathodes have been used----the so-called ZPT cathodes which operate on the principle of the interpole target, in the same arrangement as in Penning cathodes, in a double-cathode arrangement. Especially due to the greater breadth of the cathode and the special configuration of the magnetic field, the result is an increase in the bias current per unit area in comparison to a Penning cathode arrangement by a factor of 5. Also, it is possible to vary the cathode power in greater bandwidths than in Penning magnetrons. This increase is favored especially by the anode effect of the gas sprinkler which is situated in the coating chamber and is at ground potential. The increased degree of ionization is also promoted by the fact that the magnet poles and the baffle box are held at floating potential. At the same time it is important for the plasma to spread out far into the space between the cathodes. The apparently possible way to increase the ionization, by strengthening the magnetic field, cannot achieve its aim because, due to the magnetic field intensification the more intense plasma becomes compressed in front of the target surface. A lower field strength prevails in the area directly around the substrates and consequently a lower ionization.
In the curves a vanishing of the poisoned areas can be achieved by strengthening the magnetic field. The SmCo magnets are replaced by NdFeB magnets.
Despite all these measures, even in the interpole target cathode (ZPT cathode) the bombardment of the substrates by ions decreases in the marginal area more than the coating rate does, so that the quality of the coating is markedly poorer precisely in its indepth action.